Installing plugin from git branches

Currently, script/plugin install works only with the master branch of a git repository. This is fine if you want your plugin on a random edge commit, but not so fine if you want a specific release (branch or tag or commit). Right now you have to type in at least 4 commands (git clone..., cd .., git checkout..., rm -rf .git) to grab a release, this patch makes that part of script/plugin's repertoire.